Field studies of fly ash concrete structures containing reactive aggregates

Abstract

The paper presents the, findings, from field studies of fly ash concrete structures containing reactive (alkali-silica) aggregates. Data is presented from a number of hydraulic structures in Wales and Ontario constructed using geologically similar greywacke-argillite aggregates. All the structures without ash showed evidence of damage due to ASR: indeed reaction with this aggregate has led to replacement of structures in both areas. Reaction may occur at alkali levels significantly below 3 kg/m3 Na2Oe, and has been, found in a structure with a measured [available] alkali content of less than 25 kg/m3 Na20e, However; the, fly ash concrete structures are in excellent condition after more than 25 years service despite having higher alkali contents than many of the damaged structures; the Lower Notch Dam was actually constructed using a high alkali cement (> 1·0% Na20e). Microstructural and pore solution analysis showed that much of the alkali in the fly ash concrete was [bound] in the CSII and not available to the pore solution for ASR. Published findings related to the McPherson Test Road and Hanshin Expressway are reviewed; these structures have been cited as examples of ASR in fly ash concrete structures. Based on the available data, recent claims that fly ash actually [contributed] to the failure of these structures are repudiated. In fact, fly ash was found to be effective in controlling expansion in the McPherson Test Road, the cracking observed in fly ash concrete being attributed to drying shrinkage. Details regarding the use of ash in the Hanshin Expressway are not adequately reported, however, it would appear that 18% fly ash was not effective in completely suppressing expansion in this structure. This is not surprising in view of the moderate level of ash and excessive alkali content of the concrete (up to 0·33% Na20e). The balance of, field performance data sustains the concept of using Class F fly ash to control expansion due to ASR in concrete, provided the material is used at sufficient levels of replacement. Further control of the alkalis, from other sources would appear to be a prudent precaution even when the concrete contains fry ash. It is surprising that many specifications have adopted the advice, from accelerated laboratory tests and penalize fly ash by assigning an [alkali contribution] (e.g. one-sixth the total alkali or all of the ASTM C312 [available] alkali) to the material which may often preclude its use. Such advice appears to contradict the more cogent argument from the field where Class F fly ash has been successfully used for many decades with no reported incidences of ASR in structures containing sufficient levels of ash.